It has long been the practice to utilize carbon dioxide snow (dry ice) as a refrigerant for the shipment and temporary storage of poultry and meat products. Typically, a shipping box is partially filled with poultry or other perishable product and the remainder of the box is filled with carbon dioxide snow.
The basic process for generating carbon dioxide snow is relatively simple. When liquified carbon dioxide is directed through a restricted conduit to reduce the pressure from a point above the triple point pressure (approximately 75 psia) to a point below the triple point pressure, the liquified carbon dioxide is converted into a mixture of solid particles (snow) and vapor. In theory, when liquid carbon dioxide at 300 psig is flashed in the manner described, approximately 43% by weight of carbon dioxide snow is generated.
There are two basic devices employed for generating a pattern of carbon dioxide snow in devices which are adapted to fill shipping containers. The snow horn, as disclosed for example in U.S. Pat. No. 2,978,187, provides a well distributed, relatively low velocity discharge of CO.sub.2 snow but also contains large amounts of CO.sub.2 vapor in the discharge which must be vented from the packaging area. The J-tube, as disclosed for example in U.S. Pat. Nos. 3,757,367 and 4,111,671, separates most of the carbon dioxide vapor from the snow prior to discharge, but the J-tube discharge is at a relatively high velocity and the discharge is particularly difficult to confine because it is comprised of light, fine-particle snow. An advantage of the J-tube generator is that the separated carbon dioxide vapor flow can be utilized to exhaust the remaining vapor from the packing area by aspiration.
One machine for packing shipping containers with CO.sub.2 snow is disclosed in U.S. Pat. No. 3,807,187, entited "Hood and Method for Generating Carbon Dioxide Snow". There, a snow horn and a J-tube are mounted within a vapor confining hood. The vapor flow from the J-tube is directed into an exhaust which serves to remove by aspiration the carbon dioxide vapor from the snow horn discharge as well as that generated by sublimation beneath the hood. While devices of this type have proved satisfactory, certain packing applications require a more compact and efficient device capable of providing a confined, low velocity discharge.